The shaped members in question, which may or may not be of square cross-section, or in the shape of thin circular bars, may be used in the formation of certain high performance and low density composite materials which can be used in the aerospace industry.
The use of methods of pultrusion, by means of which it is known to calibrate a material by passing it through a die of precise internal cross-section, is well known for the production of threads or shaped members of composite material. In certain cases a shaped die formed by a simple diaphragm of small thickness is used, through which a staple sliver is drawn under force and calibrated after impregnation in a resin bath. In other cases, the thread preimpregnated in that manner is wound on a former of rectangular cross-section such that each preimpregnated rectilinear cord stretched across one side of the former can, after polymerization and cutting, constitute a rigid shaped composite member. Those discontinuous methods of manufacture are best used for shaped members of a length limited to the length of the support frame and therefore cannot be adapted to industrial applications that aim to produce shaped members that are calibrated with the greatest degree of accuracy and are of long length. Indeed the known methods do not allow reproducibility of shaping and cross-section since the setting of the material is effected discontinuously and is not rigorously carried out in an identical manner from one operation to another.
Pultrusion methods, comprising the calibration of a shaped member through one or more dies by means of traction of the shaped member, have allowed an improvement in the products obtained. A pultrusion bench is known generally to be constituted by a continuous unwinding station for dry threads, followed by a tank for impregnating the composite material with resin, the material then being displaced continuously by a traction mechanism inside a long heating die which ensures the shaping of the product and its polymerization. A cutting device for cutting the shaped member into lengths is provided at the outlet of that bench. Such continuous manufacture by that pultrusion method, however, requires a relatively long passing time through the heating die if complete and correct polymerization of the resin is desired without deformation of the shaped member, for example without twisting, and this is accomplished to the detriment of the speed of production. In addition, the die stands in the way of total elimination of volatile products from the resin. Finally, the existence of a single long shaping die does not eliminate completely the jamming of composite material fibrillae at the inlet, since frictional forces are associated with the length of the die and promote jamming. Whether there are several dies or one long die, these jams necessitate numerous manual interventions, especially for the purpose of cleaning, which involves stopping the machine and the loss of time in replacing the thread in the die. It is therefore not possible to use those dies for the manufacture of carbon/carbon shaped members of small cross-section since the shaped members would break.
This method of manufacture by means of a long heating die is not suitable for carbon/carbon composite material shaped members of small cross-section, that is to say, of a cross-section smaller than 1 mm. Indeed the use of that method is very tricky since the pultrusion force necessary for the shaping is significant and might cause breakage of the carbon fibers. Furthermore, the alignment of the dies is critical and it would not be possible, owing to the duration of the production, to be assured of continuous manufacture over long lengths.